Evidence for non-DLVO hydration interactions in solutions of the protein apoferritin

We have studied molecular interactions in solutions of the protein apoferri tin by static and dynamic light scattering. When plotted against the electr olyte concentration, the second osmotic virial coefficient exhibits a minim um. The ascending branch of this dependence is a manifestation of a surpris ingly strong repulsion between the molecules at electrolyte concentrations about and above 0.2M, where electrostatic interactions are suppressed. We a rgue that the repulsion is due to the water structuring, enhanced by the ac cumulation of hydrophilic counterions around the apoferritin molecules, giv ing rise to so-called hydration forces.